Do ants make slaves

In the recorded natural history of ants there are several cases of some ants taking "slaves" from neighbouring ant colonies. Let us see a few examples.

The blood red ants (Formica sanguinea), so named because of their red body colour, occur in Europe and North America. Scout workers of this species locate colonies of other ant species in neighbourhood, such as Formica fusca. An army of workers of the sanguinea colony then launches an attack on a chosen neighbouring colony. If the attacked colony workers offer resistance, they are killed. Developing stages, specially pupae, are removed by the sanguinea workers, and are taken to their own colony. Some of the pupae, brought in as trophies, are eaten, and some others are reared to their adulthood (Poole and Poole, 1963). The alien workers, thus produced in the sanguinea colony, are "slaves", and they serve the colony as if it is their own. Formica sanguinea queen is quite capable of living without its helper ants. So why this "slavery"? It seems that it is done mostly in the search of food. Only pupae when they are maturing, and never eggs, larvae or sexual ants are stolen.

While many Lasius species found their colony in the usual way independently, others may take to "slavery". The females of Lasius reginae, in Austria, founds its colony with the help of the common Lasius alienus. It is a small ant, which often penetrates into the human dwellings in Europe. The reginae female is smaller than the host L alienus queen and even smaller than her own workers. She, however, manages to enter an alienus colony and to turn the legitimate queen on her back and kill her, tearing the skin between her head and chest with mandibles. The reginae queen does not have enough reserve nourishment in her body for initiating her own colony independently. After that the new invading queen begins to lay eggs (Dumpert, 1981). The queen of Lasius carniolicus, also smaller than her workers, founds her colony with the help of Lasius flavus. A large European species, Lasius fuliginosus also practices "slavery" at the cost of Lasius umbratus.

A specially interesting case, in this context, is that of the ant Bothriomyrmex decapitans. A newly emerged queen of this species proceeds on a nuptial flight, as is common among ants. After mating she sheds her wings and enters an ant colony of another species. Moving stealthily, avoiding attention of the workers of this colony, she reaches the queen chamber of the nest. Here she climbs over the back of the reigning queen, and waits there. In this position the workers of the colony pay no heed to her. This foreign queen gradually acquires the odour of the host colony. Now she bites away the head of the queen "on throne", and kills her. The workers accept the intruder queen as their own, and remove and throw out the dead body of the original queen (Poole and Poole, 1963). The Bothri-omyrmex queen starts laying her eggs at a fast pace, and the eggs are taken care of by the host workers. Individuals of the original colony gradually die, and the Bothriomyrmex progeny take their place. Thus, after sometime the invaded colony gets transformed into a Bothriomyrmex colony.

The European ant Anergates atratulus shows the climax of specialization for the "slave" making habit. A queen of this species enters the nest of another species, a species of Tetramorium, and kills the queen of the host colony. Workers of the host colony help the foreign queen rear her progeny. It is specially notable that Anergates atratulus never makes its own nest, and is not known to produce its own workers. In contrast, in the case of Bothriomyrmex enslaving a colony of another species is facultative, that is a mated female may enter a colony of another species, or establish its own colony from the start. Another interesting situation: The Amazon ants, Polyergus rufescens, a French ant is entirely dependent on a Serviformica species. Without the latter, they are unable to obtain any food for themselves. In their colonies the "slaves" are five times more numerous than the "slave-makers".

A question, which comes up in this context: Why do those ant species, which suffer from the "slave" making habit of another species, not develop their own defences? When searching for an answer to this question, it has to be realized that to ascribe slavery to these instances is an anthropocentric thinking. In fact these are cases of social parasitism. Slavery is intraspecific, whereas the so called "slavery" among ants is mostly between different species. In cases of parasitism it is known that, when a parasite is freshly introduced into a host population, for sometime there are deep fluctuations in the host as well as in parasite population sizes. Increase in parasite population reduces the host population. Decreased host population results in a decrease in the parasite population. When the parasite population touches a certain low, host population begins to increase, and the cycle is repeated. The resulting oscillations in the sizes of the host and parasite populations are deep for some generations, and they dampen after some length of the association (Ananthakrisnan and Viswarathan, 1976), as a sort of equilibrium between the two is reached. It seems that it is this state of equilibrium between "slave" provider and "slave" maker species in the instances, described in this chapter.

Such social parasitism among ants, in which ants from one colony invade another colony and "enslave" the invaded colony, has been called "dulosis" (Holldobler and Wilson, 1990). According to Holldobler and Wilson (1990) "dulosis" in some cases may be intraspecific, that is invaders and invaded colony belong to the same species, a situation which may be seen, for example in the honey storing ant species, Myrmecoystus mimicus in USA. In this species, workers from a stronger colony drag out all pupae, larvae, and workers, including honey pots, from a weaker colony to be included in their own colony. This also may happen that a queen, unaccompanied by workers, may seek shelter in a helper colony.